The long-range objective of this research is to elucidate the number, function and regulation of the histocompatibility-linked immune response (Ir) genes and the nature of their products. We have recently produced in the mouse antisera reactive with cell-surface antigens on T-lymphocytes which are controlled by the Ir-region of the H-2 gene complex. The immediate objectives of the proposed research are to better define these new Lna antigens (lymph node antigens) and to determine their relationship, if any, to Ir gene products and to mechanisms of immune response. The specific aims of this research are: 1) to more thoroughly define the genetic and serological variation of the Lna antigens; 2) to better define the cellular and tissue distribution of the Lna antigens, especially of T-lymphocytes, and their relationship to other alloantigens on lymphocyte surfaces; 3) to define the chemical nature of the Lna antigens; and 4) to determine whether Lna antigens play a role in immunologic functions by testing the capacity of anti-Lna sera to block in vitro immune responses, in vitro stimulation of lymphocytes by specific antigens, or the mixed leucocyte reaction or cell-mediated lymphocytotoxicity.